1. Field of the Invention
The present invention relates to a variable capacitor. More particularly, the present invention relates to a structure of a radio frequency variable capacitor to improve a quality factor while using a CMOS process, and a method of manufacturing the same.
2. Description of the Related Art
In a mobile communication system, a radio frequency (RF) block is designed to support many frequency bandwidths. In particular, a variable capacitor having a different capacitance for each frequency bandwidth should be used as a capacitor used in a filter having a direct relation to a frequency bandwidth. In addition, a voltage controlled oscillator (VCO), which is one element in an RF block, adjusts a voltage applied to a variable capacitor to vary the capacitance thereof, and changes a resonance frequency thereby. In this way, the variable capacitor is a very important device for a tunable filter or a VCO for the RF block.
Meanwhile, according to a recent trend in mobile communication systems, an RF IC using a silicon process technology has been developed, and problems that are now being faced in designing the RF IC are proper modeling of a passive element at a frequency bandwidth of several tens of GHz. In particular, it is difficult to integrate a VCO using a passive element such as an inductor or a variable capacitor. That is, if the passive element is used at an RF bandwidth of several tens of GHz, a quality factor is reduced, and as a frequency increases, losses caused by a coupling effect and a skin effect with a substrate increase.
In order to solve this problem, the prior art discloses a method of implementing a variable capacitor using a microelectromechanical system (MEMS) technology. The MEMS variable capacitor uses the air as a dielectric body and thus has a high quality factor, a wide range of tuning, and an ability to be easily compensated for process or temperature variations. In addition, the MEMS variable capacitor has low phase noise, very low insertion loss and small power consumption. As such, the MEMS variable capacitor may be used in a tunable filter requiring a wide dynamic range and a VCO requiring low phase noise.
However, in spite of its advantages, the MEMS variable capacitor has a tuning voltage from at least 5V to several tens of V. Thus, it is very difficult to use the MEMS variable capacitor in an existing low power consumption element, in particular, a low voltage operating system for a CMOS and an RF communication device. Further, an additional process is required to integrate the MEMS variable capacitor into a CMOS RF IC, thereby increasing costs.